This invention relates to thermally stable labels, and more particularly, to labels especially suitable for use under conditions of high temperature and harsh chemical environments encountered in printed circuit board manufacturing processes.
Approximately seven hundred million circuit boards are currently being produced annually in North America. The market for this commodity has been growing at a rate of 7 to 10% each year. In particular, a technique for manufacturing printed circuit boards recently introduced into the United States, known as surface mount processing, has seen a dramatic upsurge during the past several years. According to industry sources, the number of printed circuit boards made using the surface mount process has more than doubled since 1987, and seems certain to continue to climb rapidly.
Surface mount processing of printed circuit boards is advantageous because of its emphasis on efficiency of manufacturing and automation. To enable circuit board manufacturers to track each board through the course of the automated manufacturing process, bar code labels are applied to the green or "raw" boards at the beginning of the manufacturing process. The manufacturer is able to maintain quality control by scanning a board's unique bar code and matching the board to the specifications assigned to that particular model, thereby ensuring that the specifications have been met. However, the surface mount process involves conditions unsuitable for conventional labels. The circuit boards are subjected to baking cycles at temperatures of 450.degree. F. and above, as well as immersion in and exposure to high pressure sprays of a variety of strong solvents. Conventional labels constructed of face stocks made from paper, polyester or polyvinyl chloride have been found to be severely limited in their ability to retain label integrity under these environmental constraints. Label curling, shrinkage, lifting, loss of scannability and related problems were frequently encountered obstacles.
Labels utilizing high temperature resistant polymer films as face stock, e.g., films which will withstand temperatures of at least 500.degree. F. indefinitely such as polyimides and polysulfones, have been commercially available since approximately 1983. However, the development of a label adhesive capable of adhering to such high temperature resistant polymers as well as to the surfaces of printed circuit boards under these conditions, without serious impairment of label integrity, has remained problematic.
Labels utilizing high temperature resistant polymer face stock in combination with acrylic adhesives made by 3M Company of St. Paul, Minn. have met with some measure of success in surviving the conditions present in the manufacture of printed circuit boards. Such labels are currently the standard of the industry for circuit board applications. However, at the higher ranges of temperatures encountered in printed circuit board processing, even labels utilizing high temperature resistant acrylic adhesives in combination with high temperature resistant polymer face stock have shown serious limitations. During extended exposure to temperatures at or above 400.degree. F. during baking cycles, as well as exposure to the harsher chemical solvents, these labels have been known to fail. Frequently encountered problems include the tendency of such labels to "float" around on, or even off, the boards under these conditions. Additionally, significant outgassing of the label adhesive may occur, resulting in loss of adhesion to the board, and/or buckling of the facestock which in turn causes loss of scannability of the label. In sum, loss of label integrity continues to be a serious problem despite the use of the improved label materials discussed above.
Further label improvement has been difficult to obtain because of the combination of features required by the demanding nature of circuit board tracking. Ideally, the label should be pressure-sensitive so that it may be applied to the board with light pressure, but have sufficiently low initial tack that it may be easily transferred and repositioned prior to circuit board processing. The label must, however, be capable of interfacing intimately with the printed circuit board to adhere even under the aforementioned conditions of high heat and exposure to solvents. Moreover, since many substances exhibit superior adhesive qualities only with certain classes of materials, the label adhesive and face stock used must be compatible.
Thus, the development of a label capable of withstanding even the most demanding requirements of printed circuit board processing is highly desirable. Moreover, the discovery of such a label has been the object of considerable interest in the industry.